Generally, printing fluid supply systems of a printer include a printing fluid supply cartridge that may be designed to deliver printing fluid to a printhead. The printing fluid supply systems may use pressurized air to pump the printing fluid from the printing fluid supply cartridge to the printhead. Alternatively, some printing fluid supply systems may include two printing fluid supply cartridges of the same printing fluid type (e.g., two printing fluid supply cartridges for each color connected via a common pipe to the printhead) to enable replacement of a printing fluid supply cartridge without having to interrupt or pause an operation (e.g., printing). In such a dual printing fluid supply cartridge scenario, it may be useful to block flow of printing fluid from an active printing fluid supply cartridge (i.e., the one delivering the printing fluid to the printhead) to the passive one, that is not delivering the printing fluid to the printhead, to avoid any undesirable refilling between the two printing fluid supply cartridges due to the printhead being located at the end of the common pipe.